1. Field of the Invention
While other applications of the present method are possible, the method is intended primarily for enhancing seismic data. In particular, the method is employed to enhance common depth point seismic data.
2. Brief Description of the Prior Art
It is conventional in seismic prospecting to arrange detectors along a line at equally spaced locations on the earth's surface. Energy sources disposed at equally spaced locations along the same line are then activated to produce seismic energy waves which spread out from the energy source in all directions. Vibrating devices and explosive devices are common examples of such energy sources. The seismic waves generated by the source are reflected, refracted and diffracted from subsurface interfaces in the earth and some of these diverted waves are received by the detectors. For purposes of brevity, reflections, refractions and diffraction will be referred to herein jointly as "reflections". The output records of seismograms produced by each of the detectors show when a diverted wave was received and give information about the time duration and intensity of the wave. These output records from the detectors can be studied to extrapolate information regarding the type and location of the subsurface formations producing the received waves. This information in turn is employed to evaluate the subsurface formations for their oil and gas bearing properties.
Because of the geometry involved, seismic waves reflected from a given subsurface point may be received by one detector from energy emitted from a first energy source and also by a second detector from energy emitted from a second energy source. This phenomenon is employed in developing common depth point (CDP) data. Because of the redundancy of information obtained in this technique, the signals may be combined or averaged so that a high signal-to-noise ratio is obtainable. In common practice, from 3 to 48 source-detector pairs are employed to develop a "gather" of CDP traces. A plurality of gathers are then combined to form a record.
A characteristic of the detector seismogram in common depth point prospecting is that the same events, i.e. trace excursions caused by reflections from the same subsurface point, tend to occur later in time on the seismogram as the distance between the energy source and the detector increases. This phenomenon is termed "normal moveout". The effect when a series of adjacent traces from a gather are assembled to form a display in which time "t" varies along the horizontal axis and detector-to-source spacing "x" varies along the vertical axis is that the reflections from common events tend to lie along a hyperbolic curve having a slope dx/dt which is an apparent velocity. This apparent velocity is termed the "stacking velocity".
It is conventional to alter the individual CDP seismic traces as required to correct for normal moveout so that the common events tend to align along a vertical line rather than a hyperbolic curve. Correction for normal moveout makes it easier for many conventional signal enhancement techniques to be employed.
In the parent of the present application, the raw seismic data from the detectors is preferably preconditioned before being enhanced by the method therein disclosed. The preconditioning techniques include correction of a gather for normal moveout followed by "stacking" which is a summing of common events on a display of CDP seismograms to produce a single resulting trace. The preconditioned data traces from a plurality of gathers are processed and assembled to form a display in which common events align in time.
While the preconditioning steps of correcting for normal moveout and stacking produce an enhanced signal which under many circumstances is very desirable, these procedures also tend to average in noise and other invalid data. Subsequent processing of the enhanced data also involves approximations and averaging so that the end result is enhanced data which is two processing phases removed from the original data. The initial step may obscure the noise and invalid information so that the second step is prevented from operating at optimum capability for signal enhancement.